Add logo to README.md

Author: acv

README.md

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 # WifiDog #
 
+[![Build Status](https://travis-ci.org/wifidog/wifidog-gateway.svg?branch=master)](https://travis-ci.org/wifidog/wifidog-gateway)
+
 The WiFi Guard Dog project is a complete and embeddable captive portal
 solution for wireless community groups or individuals who wish to open a
 free hotspot while still preventing abuse of their Internet connection.